While there are other gas flow sensors in the prior art, the present invention incorporates improvements that result in more accurate leak detection and flow measurement capabilities in a simple, easily used configuration.
One example of prior art is disclosed in U.S. Pat. No. 4,800,754 naming David M. Korpi as the inventor and Sierra Instruments, Inc. as the assignee. In the aforementioned patent, a widerange, adjustable flowmeter is disclosed. This design utilizes an adjustable laminar flow element located within a bore for fine adjustments and an adjustable, restriction secondary passage for crude adjustments. In this design, all or a selected portion of the flows to be measured can be diverted through the secondary passage. The physics of this design is based on heat transfer which is less accurate and slower to react than the technology of the present invention. Finally, by design, this apparatus is limited to low flow pressure measurements. This apparatus unnecessarily complicates the flow measurement process for various flow ranges.
Another example of prior art is disclosed in U.S. Pat. No. 4,487,062 naming John G. Olin and David M. Korpi as inventors and Sierra Instruments, Inc. as assignee. In the aforementioned patent, a mass flowmeter is disclosed. This flowmeter also includes a primary and secondary passage in addition to a pair of resistance wire coils surrounding the sensor tube whereby resistance is proportional to the fluid flow rate. Again this type of flowmeter is unnecessarily complicated.
Another example of the prior art is disclosed in U.S. Pat. No. 5,099,881 naming Takeshi Nakajima as inventor. In the aforementioned patent, the flow dividing structure of the mass flow controller includes a main mass flow passage and a bypass flow passage. Due to the conical bore design of the flow dividing structure, the cross-sectional area of the main passage cross-sectional area can be adjusted and the ratio of the areas between the main passage and bypass passage will determine the proportion of the flow passing through the bypass passage, where the measuring sensor is located. Consequently, the conical bore acts as a flow divider and not a flow sensor as in the present invention.
Another example of the prior art is disclosed in U.S. Pat. No. 5,305,638 naming Hamid Saghatchi et al as inventors. The conical portion in this invention serves a similar function as the conical portion in the Nakajima patent. As before, the conical portion is used as a flow divider rather than a flow sensor as in the present invention.
Another example of a prior gas flow measurement device is disclosed in U.S. Pat. No. 5,445,035 naming Pierre R. Delajoud as the inventor. In the aforementioned U.S. Patent, a gas flow measurement apparatus is disclosed with an elongated cylindrical piston positioned concentrically within a bore is used thus forming an annular flow channel of uniform depth. Ferrules are located at each end to position the piston. While this invention is an improvement over previous gas flow sensors, there are still disadvantages. The primary disadvantage is the two adjustments that must be made to position the piston. The device also uses a cylindrical piston which does not allow the same degree of control over the acceleration of the flow. Yet another disadvantage locates the pressure measurement port where flow is not yet filly developed, which has a tendency to adversely affect the accuracy of the measurement. Finally, this design uses two pressure sensors, one pressure sensor and one pressure differential sensor.
The main disadvantages of the prior art gas flow measurement apparatus are as follows:
1) To achieve as large a differential pressure as possible, the pressure measurement device is located where the flow is not fully developed, and hence, turbulence can occur and larger uncertainty will result in the measurement. PA1 2) The prior art cannot adjust the flow rate to the desired differential pressure and maintain the desired accuracy. This is critical for in-line flow measurements when small differential pressure is required in order not to affect the flow, e.g. leak flow measurement when the unit under test is maintained at low pressure. To increase the differential pressure resolution, acceleration/deceleration of the flow is employed in the present invention. The acceleration/deceleration is achieved by creating different cross sectional area of the gas flow at the two receiving ports of the differential pressure sensor. The cross-sectional area of the gap is the area between the ends of the bore and the shaft and is represented by the following formula: EQU A=.pi..multidot.(R+h).sup.2 -.pi..multidot.R.sup.2 =.multidot.h.multidot.(2.multidot.R+h).apprxeq.2 .pi.hR PA1 A: Cross-sectional area of the gap PA1 R: The radius of the shaft at particular location PA1 h: The gap at the particular location PA1 d: density of the gas PA1 v.sub.1 : average flow velocity at the receiving port PA1 A.sub.1 : cross sectional area at the receiving port PA1 Based on the energy conservation law, ##EQU1##
To avoid the turbulence flow phenomenon on measurement, the present invention measures the differential pressure on the section of laminar flow gap where the laminar flow is fully developed, rather than two extreme ends of the flow.
where
It is obvious that the cross sectional area will be larger if R and h are increased and vice visa. The following formula is based on the mass conservation law: EQU d.multidot.v.sub.1 .multidot.A.sub.1 =MassFlow
where
Due to the fact that the cross sectional area at the two ports is different, the velocity at the receiving port is inversely proportional to the cross sectional area if all the other parameters are kept constant.
Assuming there is no significant density and temperature change along the section of the passage, the increase of the velocity of the flow will result in a decrease in the static pressure and the amount of the pressure drop is determined by the amount of the velocity change between the two ports. Now the differential pressure is composed of two parts. The first part comes from the works by friction and shear force as Hougan Poissile Phenomena. The second part is contributed by the different velocities at the two receiving ports. As a result, the differential pressure can be controlled and is more repeatable. The gas velocity can be increased or decreased by controlling the flow direction across the conical section.
Furthermore, precisely machined spacers at the second end portion of the body can linearly position the shaft within the core to control how much the differential pressure is "Amplified".
Because the different cross sectional areas at both receiving ends is critical to this application, the machining and surface finish of the shaft and bore become more important than that in the application on a flow divider due to the ratio of the area between the main passage and bypass passage.
Besides, the prior art requires remote electronics, such as PC and data acquisition systems to interpret the signals, compute the flow and perform the control function if needed. These require higher skill from the user, and complicate the application. The present invention provides an integrated solution in a compact form. Furthermore, laminar sensors in the prior art are more difficult to clean and maintain than those in the present invention.
In summary, the present invention represents an improvement over the prior art due to the more simplistic design of the apparatus, the ease of adjustability and repeatability, the ability to accelerate/decelerate flow to create improved mathematical correlations and the lower anticipated costs of manufacturing the apparatus.